The goal of this contract is to provide support of National Toxicology Program (NTP) hazard identification activities targeted toward the prevention of diseases or adverse effects caused by environmental exposure to chemical or physical agents. The subject contract was established to provide a broad range of routine and specialized pathology services in support of studies conducted under contract by the NTP and for supplemental studies on pathology specimens generated through these studies. The contract also provides critical pathology support for studies conducted by in-house investigators at NIEHS and other NIEHS contract studies. The NTP evaluates the toxicity and carcinogenicity of agents (chemicals, physical or biological) of environmental concern by exposing rodents (usually rats and mice) to test materials by one of several different routes of administration, which include feed, gavage, inhalation, dermal application, and intraperitoneal injection. Studies designed to characterize the toxicity and carcinogenicity of these agents are conducted either at a number of testing laboratories under contract with the Environmental Toxicology Program (ETP), or at NIEHS under the auspices of the National Toxicology Program. These studies provide important data for risk assessment relating to potential human exposure to toxic substances in the environment. Pathology assessments, particularly tissue histopathological evaluations, are critical and integral components of all studies. Important goals in the complete pathological evaluation of studies performed by the NTP and NIEHS are to: 1) improve the understanding of the nature of chemically induced lesions in rodents, and 2) develop criteria and standardized terminology to facilitate consistent diagnosis, documentation and interpretation of the lesions observed, and 3) ensure uniformity, consistency, and accuracy of the diagnostic criteria and pathology procedures used. Routinely, this is accomplished through a variety of pathology tasks that are performed by qualified professionals (anatomic pathologists, molecular biologists) and technical staff (pathology assistants, histotechnologists), prior to, during, and after study completion as part of the Pathology Quality Assessment, Pathology Working Group (PWG) Coordinator and PWG panel reviews of studies, or in reviews subsequent to the PWG reviews. Furthermore, as study results become available, there is often the need to perform additional studies to further define the toxicity and/or carcinogenicity of the chemical under study, and the mechanisms that may be involved. Such studies may include additional routine gross and/or histopathological evaluations, the application of specialized procedures such as immunohistochemistry, electron microscopy, morphometrics, or the measurement of cell replication and apoptosis. In addition, studies conducted by in-house NIEHS investigators frequently require routine and/or specialized pathological support and evaluations before the studies can be completed and the results reported. These activities also require large amounts of professional and technical pathology support, and the same high standards of uniformity, consistency, and accuracy of the diagnostic criteria and pathology procedures used for generation of the data.